In recent various communication systems, an Automatic Repeat Request (ARQ) is employed as a method of correcting errors on a transmission path. For example, in a system called “Stop and Wait system”, which is one system of the Automatic Repeat Request, when a base station receives data from a terminal, the base station performs error detection after demodulation and decoding of the data. The base station then returns an ACK signal upon determining that “there is no error”, but returns a NACK signal upon determining that “there is an error”, to the terminal, respectively. After transmitting the data, the terminal waits for the ACK/NACK signal to be returned. The terminal then transmits the next new data upon receiving the ACK signal (first transmission), but upon receiving the NACK signal retransmits the data for which the NACK signal is caused (retransmission). If the base station returns the NACK signal, then the base station stores the data corresponding to the NACK signal, and upon receiving the retransmitted data, performs combining of the corresponding stored data and the retransmitted data. By performing such a combining process, it is possible to virtually increase the signal reception level and to increase the probability of receiving signals with no error. A method of enhancing the error correction capability by performing the combining process in the above manner is called Hybrid ARQ (HARQ).
By the way, because there exists a blank time (a time during which any transmission is not performed) from when the terminal transmits the data until when it receives the ACK/NACK signal in the Stop and Wait system, the system is inefficient in terms of the usage of a communication cannel. To cope with this problem, for example, in an Enhanced Uplink, which is standardized in the 3GPP (3rd Generation Partnership Project), a plurality of HARQ processes of the Stop and Wait system are performed in parallel to achieve high efficiency in the usage of the communication channel. This type of system is generally called N-channel Stop and Wait. The “N” in the N-channel means the number of HARQ processes performed in parallel, which is, in the Enhanced Uplink, eight for 2 ms TTI and four for 10 ms TTI. The TTI (Transmission Time Interval) is a time length for which one data unit subjected to error correction coding is transmitted.
In the above 3GPP Enhanced Uplink, an E-DCH (Enhanced Dedicated Channel) is defined as one of the transport channels used in an uplink channel. An E-DPDCH (E-DCH Dedicated Physical Data Control Channel) is defined as a physical channel for the E-DCH. In this system, WCDMA (Wideband Code Division Multiple Access) is adopted as a wireless access system, and a plurality of communication physical channels, E-DPDCHs are defined on a wireless channel with providing a plurality of spread codes.
Meanwhile, in the WCDMA, data transmission is generally permitted for a plurality of terminals at the same time, but there is a limit in the permission. For example, when a base station receives data from a terminal A, data from the other terminals (a terminal B and a terminal C) than the terminal A, that perform transmission at the same time becomes a cause of interference. The amount of interference increases as the number of other terminals that perform transmission at the same time increases, and if the amount of interference exceeds a certain value, it becomes impossible to maintain the quality enough to properly receive the data from the terminal A, by which reception error is frequently caused. To avoid such a phenomenon, it is necessary to control the number of terminals that are permitted to perform the transmission at the same time or the sum of transmission powers of the terminals that perform the transmission at the same time not to exceed a predetermined threshold value. For example, if a transmission request from the terminal C is generated while the terminal A and the terminal B are performing communication with the base station, then the transmission powers of the terminal A and the terminal B are reduced, respectively, and a transmission permission is given to the terminal C.
Furthermore, as a modification of the control described above, there can be a method in which a transmission power of one terminal is set to zero, and a transmission power value that has become available by this setting is granted to another new terminal. This method can also serve as a method for switching terminals to which a transmission right is granted with time. For example, this method is described as “Time and Rate Scheduling” in Non Patent Literature 1 mentioned below.
In the “Time and Rate Scheduling”, after a certain amount of time has elapsed, a terminal to which data transmission is granted is switched by instructing a resource release (Zero Grant) to a terminal to which wireless resources are allocated (given Grant) so far and giving the resources (giving Grant) to another terminal. At this time, one thing that must be considered is the above-described process of the HARQ. That is, when there are still the data to be retransmitted from the terminal A after instructing the Zero Grant to the terminal A (in a state where the base station had transmitted the NACK signal), it is desirable to issue the Grant to the next terminal B after completing the HARQ processing. When the Grant is issued to the terminal B without completing the HARQ processing, the terminal A continues the retransmission, which becomes a cause of interference to data reception from the terminal B. Furthermore, at the time of issuing the Grant to the terminal B, the base station deletes data for which the NACK signal is returned to the terminal A, from a memory buffer. Therefore, even when the terminal A is given the Grant once again after that and then curries out retransmission, the combining effect of the HARQ can not be obtained.